Negative acting photoresists exposed to activating radiation polymerize or cross-link in a reaction between photoactive compounds and polymerizable agents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in developer solutions than unexposed portions. For positive working photoresists, exposed portions are rendered more soluble in developer solutions while areas not exposed remain comparatively less developer soluble.
Negative acting photoresists, such as dry film photoresists, may be primary photoimaging resists or secondary photoimaging resists. Primary photoresists are used to form temporary coatings on substrates. Secondary photoresists are hardenable and form permanent layers, e.g., solder masks. Negative acting photoresists have various requisites such as etch resistance, heat resistance and adhesion.
Dry film photoresists include at least a resin binder, a cross-linking monomer or oligomer and a photoinitiator. A wide variety of polymeric binders may be employed in dry film photoresists. Such polymeric binders may include, as polymerized components, one or more acid functional monomers such as acrylic acid or methacrylic acid. Polymeric binders take up space in the photoresist and are passively linked to the cross-linking monomers or oligomers. Photoinitiators initiate the cross-linking reaction between the cross-linking monomers or oligomers upon exposure to light. Other additives included in photoresists are anti-striation reagents, plasticizers, speed enhancers, surfactants, fillers, and dyes.
Negative acting dry film photoresists may be laminated to a substrate. Such dry film photoresists are particularly suitable for use in printed circuit board manufacture. One problem with many primary dry film photoresists is that they are difficult to strip from metal plated circuit boards using conventional alkaline aqueous stripping solutions, e.g. 3% sodium hydroxide solutions. If the photoresist is not completely stripped and removed, ragged metal circuit lines may result which may cause short-circuiting of the board.
Many manufacturers use organic-based or organic solvent-containing alkaline stripping solutions to facilitate stripping and to produce smaller stripped particles than aqueous based strippers. Organic-based strippers also have been used for many years because they remove polymerized photoresists at faster rates than aqueous based strippers. The faster the removal rate the more efficient the manufacturing process. Stripping temperatures typically range from 55° C. to 60° C. for organic-based strippers.
While the organic-based and organic solvent-containing alkaline strippers are preferred over aqueous based strippers because of their over-all better stripping performance, such strippers are expensive relative to alkaline aqueous strippers. Organic-based strippers and organic solvent-containing strippers include more aggressive additives than aqueous alkaline strippers, such as ethylenediamine and ethylene glycol mono-butyl ether. Such additives are inherently toxic to workers and the environment and are difficult to waste treat. Organic-based and organic solvent-containing strippers typically include high concentrations of volatile organic compounds (VOCs). VOCs are organic chemicals that have high vapor pressures at ordinary room temperature conditions. Their high vapor pressure results from low boiling points which causes large numbers of molecules to evaporate or sublime from a liquid or solid form of the compound and enter the surrounding air. Many VOCs are dangerous to human health and cause harm to the environment. Accordingly, there is a need for an improved method of removing polymerized negative acting photoresists from substrates using aqueous based alkaline solutions.